As one of optical communication techniques, there is a technique for superimposing a signal, which is different from a main optical signal, on the main optical signal by means of a frequency modulation. For example, a signal associated with a monitoring or a control of an optical transmission system may be superimposed on the main optical signal by means of frequency modulation.
Related techniques are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2013-009238, and Japanese Laid-Open Patent Publication No. 2013-201495.